The rule is said to be part of a number of "format points" that producers have enforced for the second series.

Hopefuls must always be called "artists", while they must never describe themselves as "judges".

The BBC reportedly had to implement the rules as part of the £22 million two-year deal to buy the Voice format from Dutch production company Talpa.

An insider said: "Producers have given the coaches a series of strict pointers about what they can and can't say.

"One of the most important ones is they must never say that their red chairs spin - they only turn. So if Jessie hits her red button for an act - who are always called artists and never contestants - she should say, 'I turned my chair for you'.

"The red chairs only go round 180 degrees so the creators of the show say they can't be described as spinning, even though that's what most viewers say.

"It may sound silly but these format points are really important as the BBC signed up to them when it nabbed the show and the coaches have been told they can't deviate from them. The Voice prides itself on its difference to The X Factor, so coaches have been told the emphasis in their feedback to acts must be positive.

"There is no meanness here - and that comes from the fact that everyone who auditions can sing and there are no joke acts."

The four stars have also reportedly been instructed to be more enthusiastic on the BBC One show.

The source said: "The coaches have been encouraged to get out of their chairs and greet the acts after their auditions, whether they are successful or not."